Method for Preventing Vibration in Pumps

ABSTRACT

A method for preventing or reducing mechanical vibrations of a pump, in particular a centrifugal pump, during pump operation is provided. A pump controller detects at least one signal of a pump operation parameter and identifies signal fluctuations in order to detect mechanical vibrations occurring in the pump. The pump controller controls the frequency converter to modify the pump speed in order to reduce a detected pump vibration.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a method for preventing or reducing mechanicalvibrations during the operation of a pump, in particular a centrifugalpump.

Mechanical vibrations in centrifugal pumps lead to increased wear andtear and unwanted noise during operation. The causes of vibrations canbe manifold. Causes can be externally excited vibrations, for exampledue to the rotation of the pump impeller, or free vibrations due to thenatural frequencies of the built-in pump.

Free vibrations occur especially in solid pumps. Solid pumps arecentrifugal pumps for the transport of pumped media with stronglyabrasive solid parts, for example, suspensions of slag, coal or ore inmining. Occasionally, the pumped medium may also contain stones or otherrigid elements which, when hitting the pump structure, may produceshocks during pump operation which cause the free vibrations of the pumpto be excited. This effect also occurs increasingly in pumps for thewaste water sector.

A particularly unfavorable case occurs if the rotational frequency ofthe impeller, i.e. the set pump revolution rate, equals the naturalfrequency of the built-in pump or corresponds to an integer multiple ofthe natural frequency. In this case, resonance vibrations occur, i.e.the two causes of vibration mutually amplify each other. It is similarlyproblematic when the set rotational frequency of the impeller coincideswith the pipeline resonance of the conveying system.

Such a resonance case is exemplified in FIG. 1. This figure shows thefrequency response of a ready-to-use built-in centrifugal pump. Thenatural frequencies at which the system oscillates freely have thefrequency values f₁, f₂, f₃. The frequency response, i.e. the positionof the natural frequencies f₁, f₂, f₃, depends on the specific pumpstructure, the selected installation position, the materials used andthe installed bearings. If the rotational frequency of the pump wheelwhich is set by means of the frequency converter is identical to or isinstead an integer multiple of one of the natural frequencies f₁, f₂,shown, the system is excited by the externally excited rotation of theimpeller and an amplified resonance vibration of the pump occurs. If therotational frequency of the impeller is instead in the range of one ofthe anti-resonances drawn here af₁, af₂, this effect is minimal andthere is no vibration or only a very small vibration.

The idea of the present application builds on the above knowledge andproposes a method, which by targeted measures during the operation ofthe pump reduces the risk of the occurrence of possible vibrations,especially resonances, to a minimum.

This object is achieved by a method according to the features of claim1. Advantageous embodiments are the subject of the dependent claims.

For the implementation of the method, the use of a frequency converterfor changing the revolution rate of the pump is decisive. However, itdoes not matter whether such a frequency converter is integrated intothe pump, attached to the pump housing or installed separately from thepump. The same applies to the pump controller for the implementation ofthe method, which may be an integral part of the pump, but also may beinstalled as a separate unit to the pump, optionally in conjunction witha separate frequency converter.

The solution according to the invention of the present applicationconsists in varying the revolution rate by a pump controller for a pumpwith a frequency converter during the operation of the pump in such away that mechanical vibrations of the pump are reduced as optimally aspossible. Another core aspect of the invention also consists of the pumpindependently identifying its existing natural frequencies duringoperation by means of suitable signal evaluation in order to be able tooptimally adapt the set pump revolution rate based on this knowledge.

The pump therefore does not need information about its frequencyresponse which has already been generated in advance and stored in thepump but can instead determine this independently during operation. Forthis purpose, the pump records a signal during pump operation whichcharacterizes a pump operating parameter, which is influenced byoccurring mechanical vibrations. The recorded signal is subsequentlyinvestigated by the pump for the presence of any vibrations, inparticular resonance vibrations. Such a vibration is subsequentlyreduced by a suitable revolution rate change.

In the recorded signal, in particular signal fluctuations can beidentified which are caused by mechanical vibrations of the pump. Theamplitude of the identified oscillation frequency(ies) of the signal isreduced by a matching change of revolution rate. According to theadvantageous embodiment of the method, therefore, the frequency spectrumof the recorded signal is considered. It is advantageous if the signalis first transformed into its frequency spectrum by means oftransformation, in particular by means of a Fast Fourier Transformation,so as to identify the corresponding frequency values and associatedamplitudes of occurring signal vibrations.

The motor current or currents of the pump drive proves to be a suitableoperating signal for the identification of any vibrations. The currentvalues are available to the frequency converter used anyway, so that nofurther sensors are required. Since mechanical vibrations of the pumpsystem are also reflected by magnetic induction in the motor windings ofthe pump drive and accordingly in the current of the motor, the motortherefore acts as an effective sensor that can be available at any time.By appropriate current analysis, mechanical vibrations of the pumpsystem can be identified with sufficient accuracy. This possibilityexists independently of the motor type of the electric pump drive used.

As an alternative or additional operating parameter for thedetermination of the frequency-response of the pump, the pump pressureis suitable, for example, in particular the final pressure of the pump.Here, too, mechanical vibrations are reflected in the signal profile.The final pressure of the pump can be determined, for example, by meansof existing pressure sensors and can be transformed into its frequencyspectrum by signal transformations, in particular a Fast FourierTransformation.

For the signal acquisition, however, a suitable sensor does notnecessarily have to be kept available. Alternatively, for example. thecurrent pump pressure can be determined mathematically by means ofoperating point estimation. A possible method for this is disclosed inDE102018200651, the content of which is fully included at this point.

According to a possible embodiment, the method can be carried outiteratively with varying pump revolution rate, for example, to identifythat pump revolution rate at which the amplitude of an identifiedvibration is as minimal as possible. The pump thus analyzes thefrequency spectrum of the repeatedly recorded signal again after thechange of revolution rate and checks whether the variation of therevolution rate has led to a decrease in the corresponding amplitude.

The iterative implementation of the steps of the method can provide anarbitrary or random or else controlled change of revolution rate. If theamplitude increases, for example, then the change of revolution ratecarried out between two iterations is reversed, otherwise it isretained. It is also conceivable to drive continuously through a certainfull range of revolution rates and subsequently set the revolution ratewith the lowest amplitude for pump operation.

An alternative is the use of suitable methods and algorithms foridentifying a local or global amplitude minimum with the associatedrevolution rate. An interval halving method and/or an optimizationmethod are conceivable, such as an active-set method and/or a Newtonmethod, to determine as quickly as possible the appropriate revolutionrate which leads to an amplitude minimum. A genetic algorithm is alsoconceivable which, although comparatively slow, enables theidentification of a global minimum frequency response.

The setting of the revolution rate or the variation thereof during theiterations of the method also depends on which operating conditions arepredetermined, for example by the pump operator. It is conceivable, forexample, that the pump operator specifies a constant pump revolutionrate or specifies only a small tolerance range for revolution ratechanges. During the iterations of the method, a revolution ratevariation is then carried out only within the previously definedtolerance range. In such a case, an iterative implementation of themethod is usually sufficient, in which all or at least some of thepermitted revolution rates are operated at to determine thecorresponding amplitude minimum for this range.

If, on the other hand, no specification has been made by the operatorfor a permissible revolution rate range, i.e. it can instead be thefull, technically possible revolution rate range of the pump, it isexpedient if the method uses one of the aforementioned methods foridentifying the appropriate revolution rate.

According to a further advantageous embodiment of the invention,however, the method can not only serve to reduce occurring vibrations,but the determination according to the invention of the frequencyresponse is also suitable for pump monitoring, for example, to detectwear or any damage to the pump mechanism at an early stage. As hasalready been explained in detail above, a core aspect of the inventionis to determine the frequency response of the pump. This dependsessentially on the pump design, its installation position, the materialsused and the installed bearing components. A change in one of thesefactors, for example due to wear or material damage, leads to a changeof the frequency response of the pump. The pump therefore preferablystores the determined frequency response and monitors this by runningrepeating measurements for frequency shifts of the identified relevantfrequencies. If such a frequency deviation is detected, this is anindication of wear and tear or of pump damage. The pump can then producea corresponding warning message or carry out an appropriate measure.

Further investigation of the frequency change can also distinguishbetween wear and damage. Usually wear leads to a creeping change in thefrequency response, while pump damage, for example bearing damage orimpeller breakage, results in a sudden change in frequency response. Thepump therefore takes into account in its evaluation the time-relatedcomponent of the detected change to differentiate between wear anddamage. The degree of change can also be included.

In addition to the method according to the invention, the presentinvention also relates to a pump, preferably a centrifugal pump,particularly preferably a waste water pump or a solids pump or a supplypump, with an internal or external frequency converter and an internalor external pump controller for carrying out the method according to theinvention. Accordingly, such a pump is characterized by the sameadvantages and properties as have already been explained in detail abovebased on the method according to the invention. A repeated descriptionis omitted for this reason.

In addition, the use according to the invention of a pump, in particulara centrifugal pump, as a waste water pump, a solids pump or a supplypump is proposed by the application. The minimization according to theinvention of occurring mechanical vibrations is especially important forwaste water pumps or solids pumps, so that the application of the methodaccording to the invention in such pump types brings far-reachingadvantages.

Further advantages and properties of the invention are to be explainedin more detail below on the basis of an exemplary embodiment shown inthe figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: shows a possible frequency response of an installed andoperational centrifugal pump,

FIG. 2: shows a time diagram of a periodic signal and

FIG. 3: shows the calculated frequency spectrum of the time signal fromFIG. 2.

DETAILED DESCRIPTION

The invention according to the present application describes a methodfor the targeted prevention of undesirable vibration amplifications inthe resonant case during the operation of a pump, in particular a solidspump, a waste water pump or another supply pump, by means of a frequencyconverter. The foundation for the targeted prevention of these resonantvibrations is that such resonance cases must initially be detected bythe pump controller, but preferably without having to retrofit the pumpwith a special sensor system such as accelerometers. However, there isnothing to prevent fitting the pump with additional sensors, for exampleaccelerometers, which may increase the accuracy of the method ifnecessary.

Since the mechanical vibrations are a consequence of the interactionbetween the structure and the force of the motor, these mechanicalvibrations can also be seen as a superposition in the drive currents ofthe pump current of the pump drive. Since the intensity of theindividual superimposed vibrations is of interest here, the evaluationof the motor currents is carried out by analyzing the frequency spectrumof the recorded motor signal, which the pump controller obtains byexecuting the Fast Fourier Transformation (FFT).

This procedure can be briefly illustrated based on the representationsof the FIGS. 2, 3. FIG. 2 shows a time diagram of a recorded signal,which was generated here for the sake of simplicity by a superpositionof three sinusoidal signals with different frequencies. By applying theFFT, the time signal can now be decomposed into its harmonic components,and it results in the frequency amplitude spectrum represented in FIG.3, from which, as expected, the individual frequencies of the sinusoidalsignals can be read out.

Due to the FFT of the motor currents, the pump can therefore detectmechanical vibrations which are reflected in the recorded motor current.In the following step, the pump or the pump controller then seeks to setthe pump revolution rate so that the resulting rotational frequency ofthe impeller does not fall on a natural frequency of the pump or amultiple of such a natural frequency. For this purpose, the revolutionrate is initially varied and in a further step a spectrum analysis ofthe currently recorded motor current is again performed at a changedrevolution rate. If the amplitude of the occurring current oscillationhas become smaller, this is an indication that the mechanical vibrationcould be successfully reduced by the revolution rate variation. Themethod is now carried out iteratively to achieve as small an amplitudevalue of the occurring fluctuations in the current signal as possible.Finding the ideal revolution rate can in principle be carried outaccording to two scenarios:

Scenario 1: The Required Rotational Frequency is Subject to FixedRequirements.

According to scenario 1, the rotational frequency may only have acertain value. This may have energy-related reasons or the intendedpurpose requires a certain (fixed) revolution rate. In this case, thepump operator defines a tolerance value in the pump controller by whichthe circulating frequency may deviate maximally from the setpoint, forexample ±3 Hz. The pump controller then varies the revolution ratewithin the allowable tolerance range and iteratively finds out therevolution rate at which the vibration amplitude is minimal. Often evenvery small variations are sufficient to depart from the naturalfrequency of the system and thus to minimize the occurring mechanicalvibrations.

Scenario 2: There are No Special Requirements for the RotationalFrequency.

If there are no process-side requirements for the rotational frequency,the pump controller can change the pump revolution rate at will. Thisallows a targeted search for an anti-resonance and setting the finaloperating revolution rate of the pump to this anti-resonance. Theeasiest way (and thus the one with the lowest memory and processrequirements) to determine the appropriate revolution rate(antiresonance) from the available revolution rate range is based onbisection. Mathematical optimization methods are faster and moreeffective, such as the “active-set method” or the “Newton method”. Aglobal optimum can also be reliably determined by means of a geneticalgorithm.

Alternatively or in addition to the motor currents, the signal of thefinal pressure of the pump can also be examined, in that similarly tothe motor current here too the frequency spectrum is analyzed andevaluated for corresponding resonance frequencies by means of FastFourier Transformation. The final pressure can be calculated, forexample, with a pressure sensor of the pump or else by means ofoperating point estimation.

To increase the signal quality, both signals (final pressure and motorcurrent) can also be merged by means of sensor data fusion. If this isnot possible, current and pressure signals can also be evaluatedindividually. For the sensor fusion, for example the individual signalvalues can be evaluated as shown above and then merged by means ofweighting. It is also conceivable to define ranges of interest in whichthe individual results of the separately evaluated signals can beweighted differently. For example, the result of the evaluation of themotor currents for frequency ranges between 10 and 200 Hz is used, whilethe result of the final pressure evaluation for higher frequencies istaken into account.

A particular advantage of the method presented here is that the pumpitself can find its natural frequencies and therefore no mathematicalprocess model, which would be complex to develop, is required. The mainapplication of the method presented here is the prevention or reductionof vibrations to reduce wear and noise during pump operation. Inaddition, the process can also provide a contribution to wear and damagemonitoring and can warn the user in case of damage.

Wear Monitoring

With the presented method, the frequency response of the built-in pumpis permanently monitored. However, as mentioned above, this depends onthe construction of the pump, the installation position, the materialsand the bearings. Therefore a change in the frequency response is in anycase an indication that one or more of these variables have changed, forexample due to wear and tear. This information can then be used for wearmonitoring, for example in combination with the solution from DE 10 2018200 651, to which express reference is made at this point. A combinationof these two approaches makes it possible to evaluate the wear conditionmore precisely.

Warning of Damage

In contrast to wear, which leads to a very slow change in frequencyresponse, pump damage would change the frequency response abruptly andsignificantly. Damage can be, among many other things, a bearing orimpeller break. Due to the rapid change of the frequency response, thepump controller can reliably separate wear and tear and damage and inthe event of damage can issue a warning to the operator.

1-12. (canceled)
 13. A method for preventing or reducing mechanicalvibrations of a pump having a frequency converter and a pump controller,comprising the steps of: detecting with the pump controller at least onesignal of a pump operating parameter; analyzing with the pump controllerthe at least one signal to identify signal oscillations characteristicof mechanical vibrations of the pump; and changing the pump revolutionrate by the pump controller controlling the frequency converter toreduce the mechanical vibrations of the pump.
 14. The method as claimedin claim 13, wherein the step of analyzing the at least one signalincludes calculation of a frequency spectrum of the at least one signalby Fast Fourier Transformation.
 15. The method as claimed in claim 14,wherein at least one signal of the at least one signal corresponds to amotor current of a pump drive.
 16. The method as claimed in claim 14,wherein at least one signal of the at least one signal corresponds to ahydraulic final pressure of the pump, and the hydraulic final pressureis determined by one of both of a pressure sensor and an estimate of anoperating point of the pump.
 17. The method as claimed in claim 14,wherein the step of changing the pump revolution rate includesiteratively varying pump revolution rate to identify a pump revolutionrate at which an amplitude of the frequency spectrum is at a minimum.18. The method as claimed in claim 17, wherein the pump revolution rateis iteratively varied within a predefined tolerance range.
 19. Themethod as claimed in claim 17, wherein the pump revolution rate isiteratively varied to identify at least one anti-resonance of the pump,and the step of changing the pump revolution rate includes operating thepump at the at least one antiresonance of the pump.
 20. The methodaccording to claim 17, wherein the pump revolution rate is varied by oneor both of an interval halving method and an optimization method. 21.The method according to claim 17, wherein the pump revolution rate isvaried by one or more of an active set method, a Newton method, and agenetic algorithm.
 22. The method as claimed in claim 14, furthercomprising the steps of: storing the calculated frequency spectrum,comparing subsequently calculated frequency spectrums to identifyfrequency spectrum changes corresponding to changes in pump resonancevibrations.
 23. The method as claimed in claim 22, further comprisingthe step of: outputting a signal in the event of an identified change inpump resonance vibrations indicating one of both of pump wear and damageto the pump structure.
 24. A pump arrangement, comprising: a pump; afrequency converter; and a pump controller, wherein the pump controlleris configured to receive at least one signal of a pump operatingparameter, analyze the at least one signal to identify signaloscillations characteristic of mechanical vibrations of the pump, andcontrol the frequency converter to change the pump revolution rate toreduce the mechanical vibrations of the pump.
 25. The pump arrangementas claimed in claim 24, wherein the pump is a centrifugal pump.
 26. Thepump arrangement as claimed in claim 25, wherein the centrifugal pump isa waste water pump, solids pump or supply pump.